Electrodialysis



June 9, 1964 G. A. DUBEY 3,136,710

ELECTRODIALYSIS Filed Sept. '7, 1960 United States Patent 3,136,710ELECTRODIALYSS George A. Dubey, Appleton, Wis., assigner to sulphiteProducts Corporation, Appleton, Wis., a corporation of Wisconsin FiiedSept. 7, 1960, Ser. No. 54,490 4 Claims. (Ci. 204-180) The presentinvention relates generally to electrodialysis, and more particularly,it relates to more eiiicient means for electrodialyzing aqueoussolutions.

Electrodialysis is a known electro-chemical processing means fortreating solutions by the application, in a cell, of an electricalpotential between two electrodes which are usually referred to as theanode, which is positively charged, and the cathode, which is negativelycharged. Between these electrodes is disposed one or more membranes todivide the cell into a series of adjacent compartments, which arereferred to herein as frames. Upon the application of an electricalpotential between the anode and cathode, an inuent liquid iselectrodialyzed, whereby certain constituents of the liquor migratetoward the anode and other constituents toward the cathode, wherebyfractional separation of the constituents occurs. The solution in theframe adjacent the anode is generally referred to as the anolyte and thesolution in the compartment adjacent the cathode is generally referredto as the catholyte. The frame formed between the two membranes maycontain the liquor.

It will be seen that upon the application of electrical potentialbetween the anode and cathode, two solutions can be obtained from theliquid and a remaining or residual solution can be obtained from theliquor. The above arrangement in a cell has been known heretofore.However, it has not been possible, heretofore, to arrange the frames sothat more than one such series of frames can be set up between the anodeand cathode to treat solutions containing large molecular weightcompounds such as ligno-sulfonic ions and amino ions. In other words,when it is desired to recover two solutions from a solution containinglarge molecular weight ions, it has not been economically feasible totreat the solution in commercial operations.

A principal di'iculty with utilizing only three frames, i.e., an anolyteframe, a catholyte frame and a liquor frame, which together are referredtoherein as a treating frame, is the ineiciency of the operation of thesystem. In this connection, most of the resistance in an electrodialysiscell having a single treating frame occurs in the region of theelectrode, i.e., the anode or the cathode. Accordingly, to have anefficient operation it is necessary that there be many frames betweenthe electrodes to provide eiiiciency of operation.

The stacking of frames between electrodes is known in connection withthe removal of one solution from an influent. A particular example ofthis occurs in the electrodialysis of sea water to provide potable waterfrom the sea water. With such arrangement, the efficiency of the unit isgreatly increased so that it becomes an economic operation. However,such arrangements, heretofore, have been primarily used when there is tobe recovered a single solution or fractions of such single solution.

lt has not been known, heretofore, how to stack treating frames so as toefficiently treat liquors or solutions containing high molecular weightions in such manner as to recover the high molecular weight ions fromthe liquor. Accordingly, it has not been known how to treat suchsolutions as spent sulphite liquor in a practical manner. In thisconnection, spent sulphite liquor results from the digestion of wood ina cooking liquor ice in the paper making process and it includessulfonate compounds which are generally ligno-sulfonates of varyingmolecular weight, wood sugars, sulfur dioxide and basic ions, such assodium, calcium, magnesium and ammonium ions, depending upon the natureof the digestion treatment. As indicated, the ligno-sulfonates aregenerally high in molecular weight but the lower molecular weightligno-sulfonates and sugars are desirably separated from the highermolecular weight ligno-sulfonates and sugars to provide desiredfractions. It is, of course, desirable to separate out the basic andacid ions so that they can be re-utilized in the preparation ofadditional cooking liquor for the paper making process.

Accordingly, it is desirable to take the spent sulphite liquor andrecover the basic ions in the catholyte and recover ligno-sulfonates andsugars in the anolyte. As before indicated, this system is desirable,but, heretofore, has been uneconomic because of the lack of asatisfactory cell for eifecting the desired treatment.

Other solutions or dispersions which can be desirably treated aresalt-protein systems, such as milk, blood, whey, etc. In such systemsanions and cations can be separated out and protein compounds recovered.

A principal object of this invention is, therefore, the provision of aneiiicient electrodialysis cell which will provide from an iniluent orliquor containing high molecular weight ions an anion concentratingsolution or cation concentrating solution containing such ions. A moreparticular object of this invention is to provide improved apparatus fortreating spent sulphite liquor and for treating other liquors from whichit is desired to recover, efficiently, two or more solutions, one ofwhich solutions contain high molecular Weight ions. Other objects andadvantages of this invention will become apparent by reference to thefollowing description and accompanying drawing in which:

FIGURE l is a schematic drawing of an electrodialysis cell of theinvention in which three treating frames are disposed between an anodeand a cathode, from which cell three solutions are recovered, onesolution being an anion concentrating solution containing high molecularweight compounds, another solution being a cation concentratingsolution, and a third solution being a treated liquor of suitableconcentration; and

FIGURE 2 is a modified cell of the invention comprising three treatingframes disposed between an anode and a cathode, from which treatingframes are recovered a cation concentrating solution, a concentratedliquor, and three separate concentrated anion fractions.

For purposes of this application and as previously indicated, certainterms will be used in describing the invention and are defined herein.In this connection, a frame is referred to as any compartment betweentwo membranes, or between a membrane and an electrode. A treatingframeis referred to herein as a series of frames associated with theiniluent and includes the frames containing the anion concentratingsolution, the liquor and the cation concentrating solution. A treatingframe may also include frames containing fractions of the anionconcentrating and cation concentrating solutions. The term cell includesthe electrodes, the membranes and all of the frames disposed between theelectrodes. The term barrier frame, as will appear more clearlyhereinafter, is a frame disposed between treating frames.

The membranes utilized in the apparatus of the invention includeselective membranes and non-selective membranes. A non-selectivemembrane will pass both anions and cations, but may be sized to controlthe passage of certain sized molecules. The selective membranes, whichmay also be sized to pass different sized molecules, are selective inthe sense that they are charged so that anions can pass therethrough andcations are rejected, or vice versa. A cation selective membrane wlllpass cations and will not pass anlons, whereas an anion selectivemembrane will pass anions and will not pass cations. These selectivemembranes and non-selective membranes are commercially available.

This invention, in its simplest form, comprises two treating framesdisposed between an anode and a cathode, each treating frame including aliquor frame having a non-selective membrane on one side thereof. Thetreating frames are separated from one another by a barrier frame whichcontains an electrolyte for conducting current comprising anions andcations which are not deleterious to the anion and cation concentratingsolutions.

Thus, the apparatus of this invention, in its simplest form, comprises acell having a cathode, surrounding which is a cation concentrating framein which basic ions are recovered. In the simplest form of theinvention, the cation concentrating frame is also a catholyte frame.Adjacent the cation concentrating frame is a liquor frame which receivesthe influent to be treated, and which has a non-selective membrane onone side thereof. Next, adjacent the liquor frame, is an anionconcentrating frame in which the acidic ions and some normally non-ioniccompounds are recovered. These three frames comprise a lirst treatingframe. Then, adjacent this anion concentrating frame is a barrier framecontaining the electrolyte which is not deleterious to the anion orcation concentrating solutions. On the other side of the barrier frameis a second treating frame comprising a cation concentrating frameadjacent which is a liquor frame, also having a non-selective membraneon one side thereof, followed by an anion concentrating frame or anolyteframe in which is dispose the anode of the cell.

It will be apparent that by using barrier frames intermediate thetreating frames, any number of treating frames can be stacked inaccordance with this invention. This being the case, the etiiciency of acell can be greatly increased because the relative power loss caused bythe resistance around two electrodes is reduced in proportion to thenumber of treating frames stacked between the electrodes.

Of course, there needs to be particular selection or membranes betweenthe frames. In this connection, selective membranes are utilized oneither side of the barrier frames. The utilization of selectivemembranes makes sure that the anions and cations of the barrier solutiongo into the cation and anion concentrating solutions respectively andthere is no dilution of these concentrating solutions by ion transfer tothe barrier solution. One of the membranes between the liquor frame andthe anion or cation concentrating frames may be selective so that theions will travel from the liquor frame into the anion or cationconcentrating frames and not from these frames into the liquor frame.One of the membranes between the liquor frame and the anion or cationconcentrating frames should be non-selective. This is done in order toget transfer of desired large molecular weight molecules into the anionor cation concentrating solution without burning of the membrane. Aspreviously indicated, this use of non-selective membranes with theliquor frame to pass the large molecules is an important feature of thisinvention.

By suitable selection of the membranes, there can be eifected separationof an anion or cation concentrating solution into various fractions. Inthis connection, several anion or cation concentrating frames may beadjacent one another and be separated by differing membranes7 therebyobtaining different fractions of the anion or cation concentratingsolution. This will become more apparent in connection with thedescription of FIGURE 2 in the drawings hereinafter.

Now referring to FIGURE 1 of the drawings, the overall electrodialysiscell is designated by the numeral 3. In the cell 3, are a positiveelectrode referred to as the anode 5 and also in the cell is a negativeelectrode referred to as the cathode 7. Immediately adjacent to anode 5is an anion concentrating frame 9. Thus, in this instance, the anionconcentrating trarne 9 is also an anolyte frame. The anion concentratingframe is bounded 0n one side by a non-selective membrane 1I. On theother side of this non-selective membrane 11 is a liquor frame 13 whichis bounded on the other side by a selective membrane 15. The selectivemembrane 15 also provides one boundary for a cation concentrating frame17 which is bounded on the other side by a selective membrane 19. Theanion concentrating frame 9, the liquor frame 13, and the cationconcentrating frame 17 comprise together a treating frame designatedgenerally by the numeral 21. This particular treating frame is designedfor handling liquor which comprises anions having large molecularweight. i The treating frame 21 is disposed adjacent a barrier frame 23which is disposed between the selective membrane 19 and anotherselective membrane Z5. On the other side of the barrier frame 23 isanother treating frame which comprises the same elements as the treatingframe 21 and is designated by the numeral 21'. Since the treating frame21 includes the same elements as the treating frame 21, correspondingparts are indicated by the same numerals and are differentiated from theelements of the treating frame 21 by the symbol prime Adjacent thetreating frame 21 is a barrier frame which is similar to the barrierframe 23 and is designated with the numeral 23.

The apparatus illustrated in FIGURE l further includes a third treatingframe which is designated by the numeral 21 and the elements of thisframe are the same as in the treating frames 2l and 21 and aredesignated by corresponding numerals to the liquor frames 21 and 21',but are differentiated therefrom by the designation double prime It willbe readily apparent that the electrolytic cell can comprise as manytreating frames as may be desired, which liquor frames are separated bybarrier frames, as illustrated in FIGURE l.

For purposes of describing the operation of the cell, the function ofliquor frame 21 and barrier frame 23 will be described in detail, but itwill be understood that similar operations occur in liquor frames 21 and21 The liquor to be treated enters the liquor frame 13 through the line33. The cations travel through the selective membrane 15 into the cationconcentrating frame 17. The anions travel through the non-selectivemembrane Il into the anion concentrating or anolyte frame 9. The treatedliquor exits from the liquor frame through the line 35, the anionconcentrating solution leaving the anion concentrating frame through theline 37 and the cation concentrating solution leaving the cationconcentrating frame through the line 39.

The barrier solution enters the barrier frame through the line 41 andexits from the frame through the line 43.

The anion concentrating solution, the cation concentrating solution andthe barrier solution may, under certain conditions and objectives, bere-circulated, the anion solution entering the anion concentrating framethrough the line 45 and the cation solution entering the cationconcentrating frame through the line 47.

As before indicated, the treating frames 21' and 21 function in asimilar manner, as has been described in connection with the liquorframe 21. Accordingly, the influent and etiluent lines are similarlydesignated, but are differentiated by the symbols of prime and doubleprime The barrier solution desirably comprises anions and cations whichmay be utilized in the desired operation.

`Thus the barrier solution may comprise an acid such as sulphuric acid,sulphurous acid, or acetic acid with the anions travelling into thecation concentrating solution and the hydrogen ions going into the anionconcentrating solution. In order to provide the desired efliciency ofoperation, it is quite apparent that the barrier solution should be agood electrolyte and readily able to pass the current between the anodeand the cathode.

Now referring to FIGURE 2 in the drawings, this figure also discloses acell which is generally designated by the numeral 103. This cellcomprises an anode 105 and a cathode 107. The cell 103 comprises threetreating frames 121, 121' and 121".

The treating frame 121 includes a liquor frame 113 which is adjacent aseries of anion concentrating frames 109a, 109b and 109e. The liquorframe 113 is separated from the anion concentrating frames 109er, 10911,and 109C by a series `of non-selective graded membranes 111a, 111b, and111e. Between the anode 105 and the first anion concentrating frame 109eis disposed an anolyte frame 106 which is separated from the anionconcentrating frame 109e by a selective membrane 108.

The liquor frame 113 is separated from a cation concentrating frame 117by a selective membrane 115. Thus, this treating frame is also primarilyadapted to handle liquor having anionsv of high molecular weight.

The treating frame 121 is bounded by a barrier frame 123 through amembrane 119 which is a selective membrane. v

The barrier frame has a membrane 125 on the other side.

The liquor is introduced into the liquor frame `113 through the line 133and concentrated liquor leaves the liquor frame through the line 135.

The various anion concentrating solutions exit from the treating framethrough the lines 137:1, 137b and 137C. This cation concentratingsolution enters the treating frame through the line 147, and exitsthrough the line 139. The anion concentrating solution is recirculatedand enters the anion concentrating frames through lines 145a, 145b and145C. The-barrier solution enters the barrier frame through line 141 andleaves this frame through the line 143.

The treating frames 121' and 121 include the elements of the treatingframe 121 and corresponding parts of the respective treating frames 121'and 121 are diiferentiated from the parts of the .treating frame 121 bythe symbols prime and double prime respectively. The barrier frame,between the treating frame 121 and 121, is designated 123'.

. A catholyte frame 146 is disposed adjacent the cathode and isseparated from the 'cation'concentrating frame 117 by a selectivemembrane 148. The electrolyte for the anode electrolyte frame'106 iscirculated through line 110 and the electrolyte for the cathodeelectrolyte frame 146 is circulated through the line 150.

It will be seen that the cellV shown in FIGURE 2 provides five fractionsof the liquor. In this connection, the illustrated cell provides acation concentrating solution, a concentrated liquor solution, and threeanion concentrating solutions containing different fractions of theanion solution. y

The cation membranes, such as the membranes desig- A nated 15, and 15'lin FIGURE 1 and the membranes 115, 115' and 115 in FIGURE 2, should beof the cation selective type and, in this connection, particularlysatisfactory membranes are sold under the trade name Ionics CR-61. Theanion membranes, such as membranes 11, 11' and 11, in FIGURE 1, and themembranes 111a, 111:1 and 111a in FIGURE 2 could be non-selectivemembranes, as for example Cellophane 600PT or Nallilm D-30. Themembranes, such as the membranes 19 and 19 shown in FIGURE 1, and 119and 119 in FIGURE 2, are of the anion selective type, such 6 as IonicsAR-lll membranes. The membranes 25 and 25 shown in FIGURE l and themembranes 125 and should be cation selective.

The membranes shown in FIGURE 2 between the various anion concentratingframes 109a, 109b and 109C, they being membranes 111a, 111b and 111C,are graded non-selective membranes having varying pore size to providedifferent fractions of the anion solution. The pore size is adjusted topass molecules of the desired size so that fractionation may occur. Byway of example, the membrane 111a may be selected to pass moleculeshaving molecular weight of less than 5000, whereas the membrane 111b maybe selected to pass molecules having molecular weights under 1000, andmembrane 111C may be selected to pass low molecular weight ions, such asthose having molecular weights below 200.

The choice and selection of the particular membranes is within the skillof the electrodialysis art and does not comprise a part of thisinvention. The fractionation of the anion concentrating solution inseveral adjacent frames with membranes of different pore sizes is alsowithin the skill of the art.

In a particular example of this invention and referring to FIGURE 1 ofthe drawing, an ammonia base, spent sulphite liquor having a solidscontent of about 10% was used. The spent liquor had a nitrogen contentof about 4 grams per liter and included lignin material having molecularweights between about 300 and about 300,000. The cation concentratingsolution was introduced into the cation concentrating frames 17, 17' and17" and contained ammonia at a level of about 6 grams per liter based onnitrogen. The barrier frames 23 and 23 contained sulphurous acid in aconcentration of about 6 grams per liter. The anion concentratingsolution was sulphurous acid having a normality of 0.1. These solutionswere circulated through the respective frames at a rate sufficient toestablish turbulence. In operation of the cell, liquor was passedthrough the liquor frames 13 times and the aggregate residence time inthe liquor frame was one minute.

An electrical potential Was established between the anode 5 and thecathode 7 suiicient to establish a current density in the cell 3 of 30milliamperes per square centimeter of membrane area.

The impressed voltage caused the ammonia ions in the liquor to pass fromthe liquor frames 13, 13 and 13 through the cation selective membranes15, 15 and 15 into the cation concentrating frames 17, 17' and 17". Atthe same time, sulphite ions from the barrier frames 23 and 23' passedinto the cation concentrating frames 17 and 17 causing the formation ofammonium sulte and bisulite.

The membranes 11, 11 and 11" were anion selective,

and because of this only very low molecular weight compounds,particularly sugars, very low molecular weight ligno-sulfonates andsulfur dioxide, passed through the selective membranes 11, 11 and 11"into the anion concentrating frames 99 and 9". At the same time, about20 percent of the water in the liquor passed by osmosis into the anionconcentrating frames 9, 9 and 9". The higher molecular weight compoundsremained in the liquor which exited from the cell 3 through lines 35, 35and 35".

With the use of anion selective membranes as described above, theresistance of the cell increased as the selective membranes becameplugged with higher molecular weight anionic compounds until the unitbecame inoperative due to polarization with consequent overheating andburning of the membranes. When, however, non-selective membranes wereused in positions 11, 11 and 11", such plugging and destruction of themembranes did not occur.

The foregoing example described specific ow characteristics Whenoperating with the substrates and dilutions indicated. Extensiveoperation has demonstrated that 7 various concentration and dilutioneffects are obtained as the osmotic and electrodialytic conditions arealtered to meet specific requirements. Such variations are deemed to bewithin the scope of the present invention.

As another example of the operation of this invention and referring toFIGURE 2, ammonia base spent sulphite liquor having solids content ofabout l percent was introduced into the cell shown in FIGURE 2, theliquor being fed to the liquor frames 113, 113' and 113". An electricalpotential was established between the anode 105 and the cathode 107 soas to provide a current density in the cell of about 30 milliamperes persquare centimeter of area of the membrane. The feed liquor had anitrogen content of about 4 grams per liter. The cation concentratingsolution, to the catholyte frames 117, 117 and 117" included ammonia andhad a similar nitrogen content. The anion concentrating7 solutions include sulphur dioxide at a concentration of about 6 grams per liter. Themembranes 111a, 111a' and 111a were selected to pass molecules having amolecular weight of less than 3000. The membranes, 111b, 111b and 11115"were selected to pass molecules having molecular weight of less than1000. The membranes 111e, 111e and 111e were chosen to pass moleculeshaving a molecular weight less than about 200. As a result, the anionconcentrating frames 109a, 109a and 109a held a substantial portion ofthe lignin molecules having molecular weight between about 1000 and3000, whereas the anion concentrating frames 109b, 109b and 109b" heldlignin molecules having molecular weights of between 1000 and 200 andthe anion concentrating frames 109e, 109e' and 109e" primarily comprisedsulphur dioxide, sugars, and other low molecular weight compounds insolution.

The various solutions in the anion concentrating frames have particularutility in certain fields and for various purposes. In this connection,the higher molecular weight anions have excellent dispersing propertiesin oil well drilling muds, and these anions also provide unusual bindingproperties for ores. In addition, the higher molecular weight fractionsgel more rapidly either spontaneously or with the aid of gelling agentssuch as dichromate salts. The low molecular weight lignin fractions haveimproved oxidation characteristics and also have improved resin formingcharacteristics. Thus, the unit provides products with uniquecharacteristics.

In a cell having but one treating frame, about 90 percent of the currentis utilized in overcoming the resistance of the electrodes therebyproviding inefficiency of operation. On the other hand, in a cellcomprising three treating frames only about percent of the current isconsumed in overcoming the resistance of the electrodes. It is obviousthat the stacking of treating frames with the use of the barrier framesprovides a very substantially improved operation.

The various features of this invention which are be` lieved to be neware set forth in the following claims.

What is claimed is:

1. In a process for the electrodialytic treatment of spent sulphiteliquor to recover acid ions therefrom, providing ionizable aqueoussolutions in successive positions, said aqueous solutions comprising ananion concentrating solution, a spent sulphite liquor to beelectrodialyzed, a cation concentrating solution, and a barrier solutioncomcp t) prising sulphurous acid, establishing a direct currentelectrical potential across said aqueous solutions, and placingselective dialysis membranes between said solutions.

2. An electrodialysis cell comprising, an anolyte frame, a catholyteframe, at least two treating frames disposed between said anolyte andsaid catholyte frames, and a barrier frame disposed between adjacenttreating frames, each treating frame including an anion concentratingframe, a cation concentrating frame and a liquor frame disposedtherebetween, each said liquor frame having electrodialysis membranes oneach side thereof of which at least one membrane is a non-selectiveelectrodialysis membrane, said barrier frame having ion-selectiveelectrodialysis membranes on each side thereof.

3. An electrodialysis cell comprising, an anolyte frame, a catholyteframe, at least two treating frames disposed between said anolyte andsaid catholyte frames, and a barrier frame disposed between adjacenttreating frames, each treating frame including an anion concentratingframe, a cation concentrating frame and a liquor frame disposedtherebetween, said liquor frame having electrodialysis membranes on eachside thereof of which at least one membrane is a non-selectiveelectrodialysis membrane, said barrier frame having ion-selectiveelectrodialysis membranes on each side thereof, one of said ionselectiveelectrodialysis membranes permitting passage of anions into the cationconcentrating frame of one of said treating frames and the otherion-selective electrodialysis membrane permitting passage of cationsinto the anion concentrating frame of another of said treating frames.

4. An electrodialysis cell comprising, an anolyte frame, a catholyteframe, at least two treating frames disposed between said anolyte andsaid catholyte frames, and a barrier frame disposed between adjacenttreating frames, each treating frame including an anion concentratingframe, a cation concentrating frame and a liquor frame disposedtherebetween, said liquor frame having a cationselective electrodialysismembrane on the side adjacent said cation concentrating frame and havinga non-selective electrodialysis membrane on the side adjacent said anionconcentrating frame, said barrier frame having ionselectiveelectrodialysis membranes on each side thereof, one of saidion-selective electrodialysis membranes permitting passage of anionsinto the cation concentrating frame of one of said treating frames andthe other ionselective electrodialysis membrane permitting passage ofcations into the anion concentrating frame of another ot said treatingframes.

References Cited in the le of this patent UNITED STATES PATENTS1,986,920 Cross Jan. 8, 1935 2,794,776 Briggs June 4, 1957 2,810,686Bodamer Oct. 22, 1957 2,829,095 Oda et al Apr. 1, 1958 2,872,407Kollsman Feb. 3, 1959 2,878,178 Bier Mar. 17, 1959 OTHER REFERENCESBramer et al.: Industrial and Engineering Chemistry, volume 47, No. 1,pages 67-70.

Horner et al.: Industrial and Engineering Chemistry, volume 47, No. 6,pages 1121-4129.

1. IN A PROCESS FOR THE ELECTRODIALYTIC TREATMENT OF SPENT SULPHITE LIQUOR TO RECOVER ACID IONS THEREFROM, PROVIDING IONIZABLE AQUEOUS SOLUTIONS IN SUCCESSIVE POSITIONS, SAID AQUEOUS SOLUTIONS COMPRISING AN ANION CONCENTRATING SOLUTION, A SPENT SULPHITE LIQUOR TO BE ELECTRODIALYZED, A CATION CONCENTRATING SOLUTION, AND A BARRIER SOLUTION COMPRISING SULPHUROUS ACID, ESTABLISHING A DIRECT CURRENT ELEC- 